Ground water is the source of drinking water for 75 and 70 percent of the populations of Minnesota and Wisconsin, respectively, according to a study by Kroening and Andrews in 1997. In addition a 1996 report shows that for many rural communities, ground water is the sole source of drinking water.

Ground water is found in pores between particles in subsurface sediment or rock units. These areas that provide major quantities of water are termed "aquifers".

Permeable sand and gravel, deposited as glacial outwash or as alluvium, can serve as important reservoirs for ground water (sand and gravel aquifers). If sand and gravel aquifers are not overlain by a confining layer, they are termed "surficial" sand and gravel aquifers.

Surficial sand and gravel aquifers provide much of the drinking water for the Upper Mississippi River Basin study unit. These aquifers are relatively shallow and easily accessible.

Surficial sand and gravel aquifers are also areas in which contamination susceptibility is high because both water and contaminants can move quickly through these permeable materials.